The invention relates to a process for treatment of geothermal residue to produce commercially useful products such as silica from the geothermal waste and to reduce the amount of regulated and non-regulated waste resulting from geothermal power production.
Geothermal energy is a major clean energy resource. However, large scale production of energy using geothermal sources produces considerable amounts of waste in the from of residual brine and sludge. The sludge contains solids which precipitate out during the power generation process and the sludge can be highly concentrated in a variety of metal salts, many of them toxic. The sludge also contains a large proportion of silica. High disposal costs and the long-term liability associated with hazardous waste disposal are a continuing problem in the production of geothermal energy. Even if the toxic metals are removed, the sludge still requires expensive disposal. The United States Congress has enacted legislation to oversee the disposal of solid and hazardous wastes (Resource Conservation and Recovery Act (RCRA), 42 U.S.C. Sections 6921 et seq.) A major objective of the RCRA was to provide assistance to state and local governments for the management of hazardous waste. The State of California has an even more stringent hazardous waste control law. Regulations were established for the handling, processing, use, storage, and disposal of hazardous wastes.
Examples of important geothermal sites are located in California, including the Geysers in Sonoma and Lake Counties north of San Francisco and the Imperial Valley of Southern California and geothermal waste is carefully regulated by the state. Geothermal waste determined to be hazardous must be disposed of at a Class I or Class II site. Discharges of nonhazardous waste will generally be at Class III sites. Disposal of regulated waste in all the classes, particularly in Classes I and II, is quite expensive. It would therefore be advantageous to convert geothermal waste to nonhazardous and even useful products.
Geothermal fluids can include steam and hot saline solutions ranging upward to concentrated brines. These fluids can contain exceptional concentrations of dissolved solids including NaCl, KCl, silica, and metals, also hydrogen sulfide. Concentrated brines can also contain appreciable levels of heavy metals, metal salts and oxides of metals such as iron, manganese, lead, zinc, cadmium, molybdenum, thallium, chromium, titanium, antimony, nickel, bismuth, tin, arsenic, antimony and mercury and radionuclides such as radium. Silver and gold may also be present. Wastewaters or condensates from geothermal plants are often reinjected through disposal wells. However, silica and carbonate depositions can cause blockages in rock fissures necessitating chemical processing of brines before reinjection to remove these materials. Solid waste materials from geothermal plants present even more difficult problems.
The technology used to convert geothermal resources to electricity include vapor-dominated (steam) systems and liquid-dominated (hot water) systems. Vapor-dominated systems are easier to exploit for the generation of electricity because steam can be directly expanded in a low pressure turbine. However, liquid-dominated systems are more readily available. The brine from a liquid-dominated system is usually flashed, i.e., abruptly reduced in pressure, to produce steam which is then used to drive a turbine. On cooling of hot geothermal fluids, a sludge is produced which is considered a mixed waste and therefore subject to regulatory constraints. Mixed waste containing heavy metals or radionuclides requires expensive disposal in a hazardous waste site. Processing of low salinity liquids also produces a chemical residue the disposal of which is regulated. The former is associated with Salton Sea type brines and the latter is associated with the Geyser type steam condensates. Brines from the Salton Sea geothermal area in California may contain total dissolved solids up to 350,000 ppm. In other areas such as the Geysers, the major contaminants are more likely arsenic and mercury.
Geothermal power plants generate waste during well drilling and plant operation. Well drilling waste includes drilling muds, brines and residue. Operational waste includes steam condensate and sludge from condensate cooling towers and hydrogen sulfide abatement systems. The sludge is dewatered resulting in a filter cake. All of this results in large volume of both non-regulated waste and hazardous waste which requires safe disposal.
U.S. Pat. No. 5,305,607 describes a method and apparatus for separating silica from scale in a geothermal power plant using silica seed particles and mechanical means based on viscosity. Before the silica separation, metallic sulfides and other hazardous materials are first precipitated in flash crystallizers. The silica filter cake is intended for simple sanitary refuse disposal.
U.S. Pat. No. 5,098,578 describes a method for precipitating a metal from spent geothermal brine by admixing the geothermal brine with steam condensate. The method stabilizes the scale-forming constituents, identified as compounds of silica and calcium, and these are disposed of in an injection well.
U.S. Pat. No. 4,437,995 describes a method for treating geothermal brines to control the precipitation of silica. A sulfate-rich liquid is introduced into residual geothermal brine to react with barium, calcium and/or lead salts to produce a colloidal suspension which accelerates the precipitation of silica from the brine. The product is a silica plus heavy metal sulfate-rich sludge. Although the cleansed brine can be pumped into an injection well, the sludge would require toxic waste disposal.
Inventors herein have suggested the desirability of treating geothermal byproducts to produce commercially acceptable silica products, e.g., Premuzic et al., xe2x80x9cRecent Advances in Biochemical Technology for the Processing of Geothermal Byproductsxe2x80x9d, BNL 62901, April 1996; Premuzic et al., Geothermal Brines and Sludges: A New Resourcexe2x80x9d, BNL 61972, June 1995. However, the recovery of commercially valuable products is only generally mentioned in these publications without any details or specifics of how the recovery might be accomplished.
It is an object of this invention to convert geothermal wastes into non-toxic disposable materials. It is a further object of this invention to convert geothermal wastes into commercially useful products.
Commercially useful amorphous silica can be obtained by at least once contacting geothermal residue with a depigmenting reagent under depigmenting conditions to produce a mixture comprising depigmented amorphous silica-containing solids and pigment-containing depigmenting reagent liquid. The solids and liquid can be separated from each other to yield amorphous silica product. Before or after the contacting, the geothermal residue or the depigmented amorphous silica-containing solids can be subjected to treatment with a metal salts solubilizing agent to detoxify the geothermal residue by removing metals. The silica product can be neutralized and then dried at a temperature from about 25xc2x0 C. to about 300xc2x0 C. The geothermal residue most advantageously processed is a geothermal waste generated after heat extraction in the plant which contains silica and metal salts.
In one embodiment, geothermal residue is first treated with a metal salts solubilizing agent, the contacting producing a first product which includes a) pigmented amorphous silica-containing components and b) solubilized metal-containing components. The metals which are preferably solubilized and removed are toxic metals which require hazardous waste deposition such as heavy metals and radionuclides. For example, in the State of California, regulated toxic metals include antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium, silver, thallium, vanadium and zinc.
Components a) and b) are separated from each other. Component a) is generally solid and component b) is generally liquid. Therefore separation methods can be methods suitable for separating solids from liquids such as filtration, centrifugation, or sedimentation. The separation yields component a) which is predominantly pigmented amorphous silica which is then contacted with depigmenting reagent to produce a second product which includes a mixture of c) substantially depigmented amorphous silica and d) depigmenting reagent containing pigment material. The amorphous silica of c) is substantially solid while d) is substantially liquid. The c) solids and d) liquids can be: separated from each other using separation means to yield metal-detoxified, depigmented silica.
The pigments removed according to the invention include iron compounds. Other colored salts of metals may also be present, e.g., manganese, gold, silver, etc. in ppm amounts. They may contribute to the fine structural characteristics of silica.
Depending on the potential application of the product silica, an alternative to the above embodiment is also applicable. In this alternative, geothermal residue in the form of pigmented sludge is contacted directly with a depigmenting reagent without a preliminary metals solubilizing treatment. The depigmented amorphous silica produced in this alternative method generally has larger particle size and has an increased tendency to agglomerate. A comparison of 1) silica produced using a primary biochemical treatment to remove metals followed by chemical treatment to remove pigment and 2) silica produced using chemical treatment without primary biochemical treatment is shown in FIGS. 8 and 7, respectively.
The morphology, including the pore diameter, pore volume and pore area of the amorphous silica produced according to the invention, can vary with the different embodiments of the invention. In addition, depigmentation treatment alone can be used or it can be combined with biotreatment. Moreover, sequential depigmentation treatments can be used, e.g., one or more acid contacting steps at differing acid concentrations (pH). Acid contacting can be followed by neutralization with a base. A minimal acid neutralization step can be used, or the neutralization can be more severe involving dissolution in base followed by reprecipitation. The morphology of the product can also be varied through the drying temperature of the product, ranging from about 25xc2x0 C. to about 300xc2x0 C. for over time periods of up to 24 hours. According to the treatment steps used, the morphology of the product can be refined and the pore size and volume/diameter ratio can be adjusted for use in more demanding applications, e.g., chromatographic materials. The amorphous silica product can be substantially completely depigmented or can be partially depigmented. Partially depigmented silica is suitable for use in products such as fillers, rubber and rubber like polymers. Completely depigmented silica is suitable for use in products such as paper, powders, and fine chemicals.
Advantageously, the invention provides product silica suitable for clean, non-toxic landfill or for use as a feedstock for other commercial products including but not limited to fillers for paints, paper, rubber, drying agents, chromatographic materials, and other silica products including powders, catalyst supports, absorbents.